Study: Salt marshes far more resilient than previously thought

Salt marshes are resilient to sea level rise as supported by a recent Virginia Institute of Marine Science study. (Overestimation of Marsh Vulnerability to Sea Level Rise released on the 24th of February, 2016.)

Rising seas don't just endanger coastal communities like Hampton Roads — computer models show they also threaten to drown the salt marshes that are vital to the coastal ecosystem.

But a new study published Wednesday claims salt marshes are actually far tougher than those models suggest. Not only are they much less vulnerable to sea-level rise, the study says, they can, in fact, rebuild themselves even faster than seas are projected to rise.

They do so by using the very sediment and mud carried in by frequent floods and the subsequent growth of common marsh plants to build up even higher and migrate inland.

"If left to their own devices, salt marshes are miraculously resilient to sea-level rise," said Matt Kirwan of the Virginia Institute of Marine Science in Gloucester Point and lead author of the new study appearing in Nature Climate Change.

This is especially significant in Hampton Roads, which is experiencing some of the fastest rates of sea level rise in the world.

Salt marshes are wetlands that lie between land and open salt water that get flooded regularly by the tides. They've been called the important "kidneys" and "sponges" of our national water system, filtering out sediment and pollutants and soaking up excess water during heavy rain events. They supply the organic matter that serves as the base of the food chain for seafood.

Kirwan and his colleagues say salt marshes are also far more adaptable than other land types as seas keep rising at ever faster rates — a trend climate scientists expect to continue for centuries.

"You still lose your low-lying coastal forests, you lose land that used to be farmed and now it becomes salt marsh," said Kirwan. "But the salt marshes, themselves, are actually very resistant. Unlike those other types of land, they have the ability to fight back."

This ability has been underestimated, he said, because the large- and small-scale computer models that are used to project their growth are fundamentally flawed.

"These models assume that marshes rise, but only at a rate equal to recent measurements of marsh accretion," said co-author Glenn Guntenspergen. "This approach leads inevitably to marsh drowning, and predictions that most tidal wetlands will be inundated by the end of the current century."

Guntenspergen is a research ecologist with the U.S. Geological Survey in Wisconsin.

But when Kirwan and his team analyzed 186 marshes along the Atlantic coasts of North America and Europe — including a handful in Virginia — they found that, as a marsh gets more flooded, it builds at rates two to three times faster than when it wasn't flooded.

The few computer models that do allow for the dynamic feedbacks that render salt marshes so adaptable, Kirwan said, indicate their strong resilience to sea-level rise.

According to those models, marshes can generally survive 10 to 50 millimeters of sea-level rise a year. That's far higher than the current annual global average rate of 3 millimeters, and mostly exceeds the 8 to 17 millimeters per year scientists predict by 2100.

Researchers also found that a marsh — if unimpeded — won't drown, but migrate to adjacent uplands.

"This is really counterintuitive," Kirwan said, "but some salt marshes will be bigger with sea level rise, because, if they survive where they are and they migrate inland, then that allows them to maybe even expand a little bit.

"We see places all around Chesapeake Bay, and really up and down the coast of North America, where that's actively occurring. If you go to some of the lower parts of Gloucester County, you can see lots of dead trees adjacent to the marsh, and there's new marsh forming underneath all those dead trees. So that's a sign that the marsh is moving inland."

That resilience falters, however, if a marsh is "hardened" by coastal cliffs or by man-made barriers such as 7riprap, sea walls or development, he said. The Chesapeake Bay has about 300,000 acres of tidal wetlands, and about 20 percent of that is already hardened.

In Louisiana, which is experiencing the highest rate of sea-level rise in the country, he said, they're trying to re-engineer portions of the Mississippi River to build up salt marshes in an effort to protect their coastal infrastructure.

And low-lying coastal countries such as the Netherlands, Belgium and the United Kingdom that previously built dikes to protect against coastal floods are now engineering salt marshes to protect their dikes, he said.

Other co-authors of the study include Stijn Temmerman of the University of Antwerp, Emily Skeehan of VIMS and Sergio Fagherazzi of Boston University.

VIMS is affiliated with the College of William and Mary in Williamsburg.